1. Field of the Invention
This invention relates to cellular telephone systems and, more particularly, to processes for designing and improving the performance of cellular telephone systems.
2. History of the Prior Art
Presently available commercial mobile communication systems typically include a plurality of fixed base stations (cells) each of which transmits signals to and receives signals from mobile units within its communication area. Each base station is assigned a plurality of channels over which it can communicate with mobile units. A mobile unit within range of the base station communicates with the external world through the base station using these channels. Typically, the channels used by a base station are separated from one another sufficiently that signals on any channel do not interfere with signals on another channel used by that base station. To accomplish this, an operator typically allots to a base station a group of channels which are each widely separated from the next. So long as a mobile unit is within the area in which the signal from a base station is strong enough and is communicating with only that base station, there is no interference with the communication.
In order to allow mobile units to transmit and receive telephone communications as the units travel over a wide geographic area, each cell is normally physically positioned so that its area of coverage is adjacent to and overlaps the areas of coverage of a number of other cells. When a mobile unit moves from an area covered by one base station to that covered by another, communication with the mobile unit is transferred (handed off) from one base station to another in an area where the coverage from different cells overlaps. Because of this overlapping coverage, the channels allotted to the cells are carefully selected so that adjoining cells do not transmit or receive on the same channels. The channels used by adjoining base stations are also theoretically separated from the channels of each adjoining base station sufficiently that signals from any base station do not interfere with signals from another adjoining base station. This separation is typically accomplished by assigning a group of widely separated non-interfering channels to some central cell and then assigning other groups of widely separated non-interfering channels to the cells surrounding that central cell using a pattern which does not reuse the same channels for the cells surrounding the central cell. The pattern of channel assignments continues similarly in the other cells adjoining the first group of cells. The pattern is often called a channel reuse pattern.
So long as a mobile unit is within the area in which the signal from a base station is strong enough and is communicating with only that base station, there is no interference with the communications. However, when a mobile unit moves from an area covered by one base station to that covered by another base station, the communication must be transferred from one base station to the other in an area. This requires cell coverage to overlap. Because of this overlapping coverage, the channels allotted to the cells are carefully selected so that adjoining cells do not transmit or receive on the same channels.
There are a number of different types of mobile communications systems. Channels are defined in different manners in each of the different systems. In the most prevalent American Mobile Phone System (AMPS) system, channels are defined by frequency. A frequency band of 25 MHz providing approximately four hundred different adjoining FM frequency channels is allotted by the federal government to each cellular operator. In a typical AMPS system, each channel uses a fixed FM frequency band width of 30 KHz. for downlink transmission from a base station to a mobile unit and another fixed FM frequency band width of 30 KHz. for uplink transmission from a mobile unit to a cell. Typically, the frequencies assigned to the downlink transmissions for an entire cellular system immediately adjoin one another and are widely separated from the frequencies assigned to the uplink transmissions which also immediately adjoin one another. In this specification, even though widely separated, the pair of frequencies used for both downlink and uplink transmission are generally intended when reference is made to an AMPS channel unless the context indicates otherwise.
Since channels are defined by frequency in an AMPS system, the channels used by any single base station are separated from one another in frequency sufficiently to eliminate interference between those channels. An operator typically allots a base station a set of channels with frequencies which are each separated from the next by some large number (e.g., twenty-one) channels carrying intermediate frequencies. Thus, in a system with twenty-one channel separation, one base station might use channels 1, 22, 43, 64, 85, and so on up to a total of between five and one hundred individual channels.
When a mobile unit moves from an area covered by one base station to that covered by another base station in an AMPS system, the communication must be transferred from one base station to the other in an area in which cell coverage overlaps. Because of this overlapping coverage, the channels allotted to the cells are carefully selected so that adjoining cells do not transmit or receive on the same frequencies. This is typically accomplished by assigning channels to a central cell which are widely separated in frequency in the manner described above, and then assigning channels to the cells surrounding that central cell using a pattern which increases each channel number by one for each sequential cell surrounding the central cell. Thus, if cells are arranged in a honeycomb pattern in which six cells surround a central cell using the above-described channels, a first cell adjacent to the central cell may have channels 2, 23, 44, 65, 86, and so on while a second cell adjoining the central cell may have channels 3, 24, 45, 66, 87, and so on. The pattern of channel assignments continues similarly in the other cells adjoining the central cell.
In some AMPS systems, especially those with cells in urban areas carrying heavy traffic, each cell may be further divided into two or three sectors each of which may include channels having the above-described frequency allotment of channels. The antennas of each sector are typically arranged to provide 180 or 120 degree coverage. When cells are discussed herein, sectors are normally meant as well unless the context indicates otherwise.
Another type of mobile system called Code Division Multiple Access (CDMA) uses digital signals to transmit data. All of the base stations of a CDMA system use the same xe2x80x9cspread spectrumxe2x80x9d frequency band of 1.25 megacycles to transmit the digital signals. The transmissions are combined with redundant channel coding information to allow error correction. The encoded signals are then multiplied by one of sixty-four Walsh codes which establish individual channels and increase the bandwidth to 1.25 megacycles. Because of the redundancy of the encoded signals, a receiver may decode a signal from the plethora of coded channels carrying data on the broad frequency band. Since the Walsh codes establish a number of individual channels and the pseudo-noise code assigned to each base station differs from those of other surrounding base stations, adjacent and remote cells may reuse the same frequency bands.
In another common type of mobile system called Time Division Multiple Access (TDMA), frequencies are assigned to the entire system in groups much like they are assigned in an AMPS system. However, within any frequency, each base station sends and receives in bursts during some number of different intervals or time slots. These time intervals within frequency bands then effectively constitute the individual channels. By assuring that the group of frequencies assigned to any individual base station differ from one another and from the frequencies assigned to base stations surrounding each individual base station, a channel reuse pattern is established which allows substantially greater use of the frequency spectrum because of the time division process.
In theory, these forms of cell arrangement and channel assignments allows channel reuse patterns to be repeated at distances separated sufficiently to negate interference between mobile units on the same and adjacent channels.
Unfortunately, interference does occur for a number reasons. Antenna patterns, power levels, scattering, and wave diffraction differ from cell to cell. Buildings, various other structures, hills, mountains, foliage, and other physical objects cause signal strength to vary over the region covered by a cell. Consequently, the boundaries at which the signal strength of a channel falls below a level sufficient to support communications with a mobile unit vary widely within a cell and from cell to cell. For this reason, cells adjacent one another do not, in fact, typically form the precise geometric boundaries suggested above. Since cell boundaries must overlap to provide complete coverage of an area and allow handoff and because the boundaries of cells are imprecisely defined, signals will often interfere with one another even though they are generated by cells which are at distances theoretically sufficient to eliminate interference. This is especially true when a sectored cell pattern is used because the cells are much closer to one another than in a simple cell pattern.
A first signal on a channel from a remote cell interferes with a second (usually) stronger signal carrying a mobile transmission on the same channel within the coverage area of a cell when the drop in strength of the first signal from the second signal is less than some threshold level (typically measured in decibels). A signal from another cell on a channel at a frequency adjacent the frequency of a channel carrying a mobile transmission interferes when the drop in strength of the interfering signal from the serving signal is less than some second threshold level. The values are determined by the particular type of mobile system involved. For example, in an AMPS system, a signal on the same channel (co-channel) from a remote base station interferes with a desired carrier signal if the interference level is not 18 dB lower than the desired carrier; and a signal on an adjacent channel from another base station interferes with a desired carrier signal if the interference level is not 6 dB lower than the desired carrier. For a CDMA system, an interfering signal must be more than 14 dB stronger than the carrier to obscure a carrier signal because the codes establishing the channels establish heavily redundant signals from which patterns may be extracted even though the interfering signal is stronger.
In order to determine whether interference exists, a mobile system operator typically relies on customer complaints. When customers register a sufficient number of complaints regarding communication at particular points in a system, an operator will usually conduct a relatively expensive field test of the suspected portion of the system to measure carrier signals and interference received. During the test, the portion of the system in which the tests are conducted is essentially disabled. Because of the expense and inconvenience, the tests are typically limited only to the suspected area. Because such tests are limited to determining the interference at those points at which a system operator expects to find interference, the efficacy of these tests is very suspect.
The tests provide data from which the points at which channels from different cells actually interfere with one another may be determined. If the level of interference is sufficiently large, the operator may change the channel group assigned to the particular area. That is, the frequency group assigned to a cell (or cells) may be changed in its entirety to another frequency group in which channels which would interfere with channels carried by other cells do not exist. It is also possible to eliminate some interference by changing cell characteristics (such as antenna tilt or power used in particular cells) without changing the channels used. Once channels have been assigned to cells which provide acceptable coverage and detected interference has been eliminated, the system is fixed and operated until other complaints arise.
A major problem with the process is that it does not provide a complete understanding of interference which actually exists in a system since typically only those positions at which extensive interference has been reported are tested for actual interference. The process does not take into consideration all of the possible signals which might be propagating into the affected area to interfere with the carrier nor does it take into consideration the effects which a change in channel assignments may have in other areas of the system. Often (and possibly usually) this method of curing interference merely exports the interference to another portion of the system where it is only discovered when a sufficient number of complaints arise to warrant a field test of the newly isolated area of interference.
Moreover, this method of placing cells, assigning frequencies, and eliminating interference is quite slow and labor intensive. Testing a medium sized system may require as much as 400 man hours. The process greatly increases the costs of creating and maintaining mobile systems without guaranteeing that interference will be eliminated. Because of the emerging nature of the market for cellular telephones, system changes which cause interference such as traffic growth are taking place constantly and at an accelerating rate. Complicating the general problem of interference in an existing system is the fact that cellular system operators are presently installing new CDMA and TDMA systems because they allow a greater number of mobile units to utilize a system and because these digital system provides a better quality of service when they are functioning properly. Often the installation of these new systems is taking place where AMPS cellular systems already exist and will continue to exist. In general, with these systems, some of the frequencies used in the AMPS systems are removed; and a CDMA base station is positioned in place of a sector at a base station.
It is desirable to provide a process by which the quality of service provided by a cellular system (and portions thereof) may be determined in terms of fixed verifiable quantities so that changes may be made to enhance the quality of service with an expectation that the changes will have the desired result in actually improving the quality of service provided by the system.
The present invention is realized by a computer implemented process which compares signals communicated between a known position and a plurality of base stations in a cellular telephone system to determine the level of interference with a signal on a channel expected to serve the known position, and determines a value indicating a probability of interference with a signal on a channel expected to serve the known position.
In one embodiment, changes in the system to improve the interference value are implemented only if the interference value is above a certain level.